Just before the release of the IPCC’s AR6 WGIII report (Mitigation of Climate Change) Joeri Rogelj had a Carbon Brief guest post on how not to interpret the emission scenarios in the IPCC report. It might have been to try and pre-empt some of the simplistic narratives that some have promoted about emission scenarios, but I can’t be sure about that being the motivation.
The key point being made was that
Scenarios can be thought of as stories of what could happen in the future. What they are not, it is important to note, are forecasts or predictions for the future.
As the article goes on to say, scenarios are essentially “what if” thought experiments that help us to answer some questions about what might happen in the future. They do not, however, encompass all possible futures, and are not predictions of the future.
However, the issue of scenario plausibility has been a bit of a hot topic and is something I’ve been thinking about a bit myself. I haven’t managed to really draw any strong conclusions, but the beauty of a blog is that I can present some ideas that others can challenge/clarify in the comments.
I tend to think that scenarios should be plausible in the sense of not violating some fundamental laws of physics, or the essentials of chemistry. I don’t necessarily think they need to pass some kind of societal plausibility test, at least in the sense of them being what we will probably do. They could be “what ifs”, or “what could have been” or even “what might be”. Of course, I do think the motivations, and assumptions, should be made clear when communicating the results of any analysis based on a particular scenario, but I don’t think they have to pass some kind of societal plausibility test.
However, one of the key talking points about scenarios is whether or not their plausibility should be more explicit. I can see some merit to this, but I can also see why this may not be appropriate. If scenarios are “what if” thought experiments, rather than forecasts or predictions, then we should be cautious of suggesting that they are more the latter than the former.
There’s also the issue of independence. If the point of scenarios is to understand the impact of various possible futures, then assigning something like a probability to a scenario might then influence the outcome. For example, if we claim that a particular scenario is impossible, we may then either give up trying to achieve it, or assume that we no longer need to do anything to avoid it. In a sense, the probability of a scenario emerging in reality could be influenced by the probability assigned to that scenario, which would then undermine the intention of these being policy relevant, rather than policy prescriptive.
A potentially interesting issue, though, is the self-consistency of scenarios. In most cases a scenario is developed and then that scenario is used as input to some kind of model to assess the potential impact of that scenario. However, rarely do these models then include how these impacts might influence the scenario itself.
This can have a number of potential consequences. It could be that the impact of following a particular scenario could be so severe that it essentially precludes anything like that scenario from actually emerging. On the other hand, if we don’t consider how the impacts might influence the scenario itself, we might conclude that everything will still essentially be fine even under some extreme scenarios. So, I think it’s important to make clear that these scenarios are often not really self-consistent.
As usual, this has got rather long. I do think we should mostly treat scenarios as “what ifs” and should let society decide what to do, or not do, given what these scenarios might be suggesting. Of course, I do think we should be very clear about the motivations, and assumptions, behind these scenarios, but – IMO – we should be cautious about explicitly defining the plausibility of these scenarios. However, I would also be interested to know what others think.